1. Field of the Invention
The present invention relates to a crystalline thin-film semiconductor and a manufacturing method thereof. The invention also relates to a semiconductor device using the above thin-film semiconductor and a manufacturing method thereof.
2. Description of Related Art
Techniques are known in which a crystalline silicon film is formed on a glass or quartz substrate and thin-film transistors (hereinafter referred to as TFTs) are formed by using the silicon film. Such TFTs are called high-temperature polysilicon TFTs or low-temperature polysilicon TFTs.
In the case of high-temperature polysilicon TFTs, a crystalline silicon film is formed by a technique including a heat treatment at a relatively high temperature of 800-900xc2x0 C. It can be said that this technique is derived from an IC manufacturing process using a single crystal silicon wafer. Naturally, high-temperature polysilicon TFTs are formed on a quartz substrate, which withstand the above-mentioned high temperature.
On the other hand, low-temperature polysilicon TFTs are formed on a glass substrate, which is inexpensive but is apparently lower in heat resistance than a quartz substrate. To form a crystalline silicon film for low-temperature polysilicon TFTs, heating at lower than 600xc2x0 C. which a glass substrate can withstand or laser annealing which causes almost no thermal damage on a glass substrate is performed.
The high-temperature polysilicon TFT is advantageous in that TFTs having uniform characteristics can be integrated on a substrate.
On the other hand, the low-temperature polysilicon TFT is advantageous in that a glass substrate can be used which is inexpensive and can easily be increased in size.
According to the current manufacturing techniques, there are no large differences in characteristics between the high-temperature polysilicon TFT and the low-temperature polysilicon TFT. That is, in both cases, the mobility is 50-100 cm2/Vs and the S value is 200-400 mV/dec. (VD=1 V).
However, these values are much worse than those of MOS transistors formed on a single crystal silicon wafer. In general, the S value of MOS transistors formed on a single crystal silicon wafer is 60-70 mV/dec.
At present, there are active matrix liquid crystal display devices in which an active matrix circuit and peripheral driver circuits are integrated on the same substrate by using TFTs. In this type of configuration, the source driver circuit of the peripheral driver circuits is required to operate at a frequency higher than a little more than 10 MHz. However, at present, a circuit using high-temperature polysilicon TFTs or low-temperature polysilicon TFTs can provide a margin of operation speed that is as small as several megahertz.
For this reason, at present, a liquid crystal display device is constituted by dividing its operation (called xe2x80x9cdivisional drivingxe2x80x9d). However, this method has several problems; for example, stripes appear on the screen due to, for instance, a slight deviation in the division timing.
It is now considered a configuration in which not only peripheral driver circuits (constituted of a shift register circuit and a buffer circuit) but also an oscillation circuit, a D/A converter, an A/D converter, and digital circuits for various kinds of image processing are integrated on the same substrate.
However, the above-mentioned oscillation circuit, D/A converter, A/D converter, and digital circuits for various kinds of image processing are required to operate even at higher frequencies than the peripheral driver circuits. Therefore, it is very difficult to constitute such circuits by using high-temperature polysilicon TFTs or low-temperature polysilicon TFTs as long as they are formed by the current manufacturing techniques.
On the other hand, integrated circuits of MOS transistors formed on a single crystal silicon wafer which circuits can operate at more than 100 MHz have already been put to practical use.
An object of the present invention is to provide a TFT which can constitute a circuit that is required to perform a high-speed operation (generally at more than tens of megahertz).
Another object of the invention is to provide a TFT whose characteristics are equivalent to those of a MOS transistor formed on a single crystal silicon wafer. It is also intended to provide a means for manufacturing such a TFT. It is further intended to provide a semiconductor device having a required function by using TFTs having so superior characteristics.
According to one aspect of the invention, there is provided a semiconductor device using a thin-film transistor that uses, as an active layer, a crystalline silicon film formed on a substrate having an insulating surface, wherein the crystalline silicon film has a crystal structure that is continuous in a predetermined direction, and grain boundaries extending in the predetermined direction; and the predetermined direction is at a predetermined angle with a direction connecting a source region and a drain region of the thin-film transistor.
FIGS. 6 and 7 show an example of a crystalline silicon film having the above-mentioned crystal structure. FIGS. 6 and 7 are photographs of obtained by observing the surface of a 250-xc3x85-thick crystalline silicon film with a transmission electron microscope (TEM). FIG. 7 is an enlargement of part of the photograph of FIG. 6.
The crystalline silicon film of FIGS. 6 and 7 can be obtained by a manufacturing process of a first embodiment of the invention which will be described later.
FIGS. 6 and 7 show a crystal structure that continuously extends from the bottom left to the top right in these drawings, as well as many grain boundaries extending substantially parallel with the above direction.
As is apparent from the crystal structure shown in FIG. 7, this crystalline silicon film is a collection of many crystallizations (crystalline silicon grains) each having a crystal structure extending in the particular direction. The width of the crystallizations is 500-2,000 xc3x85, or from about the thickness of the crystalline silicon film to 2,000 xc3x85.
Many definite grain boundaries are arranged, at intervals, perpendicularly or substantially perpendicularly (in the direction from the bottom right to the top left in these drawings) to the direction in which the crystal structure has continuity; the crystal structure is discontinuous (continuity is lost) in the former direction.
The continuity of the lattice structure is substantially maintained in the direction in which the crystal structure has continuity. In this direction, the scattering and trapping of carriers during their movement occur at a much smaller possibility than in the other directions.
That is, it can be considered that a substantial single crystal state, in which carriers are not scattered or are hardly scattered by grain boundaries, is established in the direction in which the crystal structure has continuity.
The above-mentioned aspect of the invention defines the relationship between the direction in which the crystal structure has continuity and the direction connecting the source and drain regions of a thin-film transistor. To attain a high-speed operation, it is desired that the direction in which the crystal structure has continuity coincide or substantially coincide with the direction connecting the source and drain regions. This provides a configuration in which carriers can move most easily.
The characteristics of a thin-film transistor can be controlled by setting the angle between the above two directions at a proper value. For example, in the case of forming a number of thin-film transistor groups, the characteristics of a plurality of groups can be made different from each other by changing the angle between the two directions from one group to another.
A thin-film transistor in which the active layer is bent to assume an N-like or a square-bracket-like, or even an M-like shape, that is, the line connecting the source and drain regions is bent can be formed in the following manner. That is, the direction in which the crystal structure has continuity is so set as to coincide with the carrier moving direction (as a whole) in the channel region.
Also in this case, the fastest operation can be expected when the angle between the carrier moving direction and the direction in which the crystal structure has continuity is set at 0xc2x0. It is apparent that this angle may be set at a proper value other than 0xc2x0, when necessary.
According to another aspect of the invention, there is provided a semiconductor device using a thin-film transistor that uses, as an active layer, a crystalline silicon film formed on a substrate having an insulating surface, wherein the crystalline silicon film is anisotropic in a grain boundary extending direction; and the predetermined direction is at a predetermined angle with a direction connecting a source region and a drain region of the thin-film transistor.
According to still another aspect of the invention, there is provided a semiconductor device using a thin-film transistor that uses, as an active layer, a crystalline silicon film formed on a substrate having an insulating surface, wherein the crystalline silicon film is anisotropic in a grain boundary extending direction; and the grain boundary extending direction and a carrier moving direction in a channel region of the thin-film transistor form a given angle.
To obtain the crystalline silicon film of the invention, it is necessary to perform a heat treatment after introducing a metal element that accelerates crystallization of silicon as typified by nickel into an amorphous silicon film, and further perform a heat treatment in an atmosphere containing a halogen element.
As the metal element, nickel is the best in terms of reproducibility and effects. In general, the metal element may be one or a plurality of elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
Where nickel is used, the concentration of nickel left in a final silicon film is 1xc3x971014 to 5xc3x971018 atoms/cm3. If the gettering conditions of a thermal oxidation film are refined, the upper limit of the concentration can be reduced to 5xc3x971017 atoms/cm3. The concentration can be measured by the SIMS (secondary ion mass spectrometry).
In general, the lower limit of the nickel concentration is 1xc3x971016 atoms/cm3. This is because when a balance with the cost is considered, it is usually difficult to eliminate the influences of nickel attached to a substrate and an apparatus used.
Therefore, when an ordinary manufacturing process is employed, the concentration of residual nickel is in a range of 1xc3x971016 to 5xc3x971017 atoms/cm3.
Since metal element moves into a thermal oxidation film in the step of forming it, metal element has a concentration gradient or distribution in the thickness direction of a resulting crystalline silicon film.
It is generally observed that the concentration of the metal element in the crystalline silicon film increases toward the boundary of the thermal oxidation film. Under certain conditions, it is observed that the concentration of the metal element increases toward a substrate or an undercoat film, i.e., toward the lower-side boundary.
Where a halogen element is contained in an atmosphere used in forming the thermal oxidation film, the halogen element assumes a concentration distribution similar to that of the metal element. That is, the concentration increases toward the upper surface and/or lower surface of the crystalline silicon film.
In the invention, it is preferred that the crystalline silicon film finally has a thickness of 100-750 xc3x85. It is even preferred that the thickness be 150-450 xc3x85. With the thickness in such ranges, the unique crystal structure in which the crystallinity is continuous in one direction as shown in FIGS. 6 and 7 can be obtained with high reproducibility in a more enhanced manner.
It is necessary to determine the thickness of the final crystalline silicon film by considering the fact that the thickness is decreased by the formation of the thermal oxidation film.
According to a further aspect of the invention, there is provided a manufacturing method of a semiconductor device, comprising the steps of forming an amorphous silicon film on a substrate having an insulating substrate; selectively introducing a metal element for accelerating crystallization of silicon into a portion of the amorphous silicon film; performing a heat treatment, to thereby effect crystal growth parallel with the substrate in a direction from the portion in which the metal element has been introduced to the other portions; performing a heat treatment at 800xc2x0-1,100xc2x0 C. for 30 minutes or more by an electrical furnace in an oxidizing atmosphere containing a halogen element, to form a thermal oxidation film; removing the thermal oxidation film; and arranging a direction connecting a source region and a drain region of the semiconductor as to coincide with or substantially coincide with a crystal growth direction.
The crystalline silicon film of the invention can be obtained by the above manufacturing process. Further, a MOS thin-film transistor utilizing the uniqueness of its crystal structure can be formed.
A metal element can be introduced by various methods, among which are a method of applying a solution containing the metal element, a method using CVD, a method using sputtering or evaporation, a plasma processing method using an electrode that contains the metal element, and a method using gas absorption.
To introduce a halogen element, there may be used a certain means for causing an oxidizing atmosphere (for instance, an oxygen atmosphere) to contain HCl, HF, HBr, Cl2, F2, Br2, CF4, or the like.
It is effective to introduce a hydrogen gas into an atmosphere used in forming a thermal oxidation film, and thereby utilize the action of wet oxidation.
The temperature is a very important factor in forming a thermal oxidation film. To obtain a TFT that can operate in itself at more than several tens of megahertz and has a small S value of less than 100 mV/dec. (described later), it is preferable that the temperature of a heat treatment for forming a thermal oxidation film be more than 800xc2x0 C. It is even preferable that the temperature be more than 900xc2x0C.
The lower limit depends upon a pressure at which the annealing is performed and a vapor pressure of the halogen compound of the material to be gettered. That is, when the vapor pressure of the halogen compound is smaller than the pressure of the annealing atmosphere, the gettering efficiency is not so high. For example, the vapor pressure of nickel chloride is 38.9 mmHg at 541xc2x0C. and 820.6 mmHg at 994xc2x0 C. Accordingly, when the annealing is performed at the atmospheric pressure (760 mmHg), the effect of the gettering is significantly increased when the temperature is 994xc2x0 C.
It is appropriate that the upper limit of the above temperature be set at about 1,100xc2x0 C, which is the upper limit a quartz substrate withstands.
According to another aspect of the invention, there is provided a manufacturing method of a semiconductor device, comprising the steps of forming an amorphous silicon film on a substrate having an insulating substrate; selectively introducing a metal element for accelerating crystallization of silicon into a portion of the amorphous silicon film; performing a heat treatment, to thereby effect crystal growth parallel with the substrate in a direction from the portion in which the metal element has been introduced to the other portions; performing a heat treatment at 800-1,100xc2x0 C. for 30 minutes or more by an electrical furnace in an oxidizing atmosphere containing a halogen element, to form a thermal oxidation film; removing the thermal oxidation film; and arranging a carrier moving direction in a channel region as to coincide with or substantially coincide with a crystal growth direction.
The above manufacturing method pays attention to the carrier moving direction in the channel region, and defines the relationship between the carrier moving direction and the crystal growth direction (i.e., the direction in which the crystal structure has continuity or grain boundaries extend).
This manufacturing method is effective even in a case where the line connecting the source and drain regions is bent.
A specific example of the invention will be described below. In the technique of forming a crystalline silicon film by crystallizing an amorphous silicon film by heating it, a heat treatment is performed in a state that nickel is held in contact with a portion of the surface of the amorphous silicon film, so that crystal growth proceeds parallel with a substrate from the above portion to the other portions.
Subsequently, a thermal oxidation film is formed on the surface of a resulting crystalline silicon film by performing a heat treatment at 800-1,100xc2x0 C. for 30 minutes or more by an electrical furnace in an oxidizing atmosphere containing a halogen element.
The thermal oxidation film is then removed. A crystalline silicon film thus obtained has a structure in which grain boundaries extend in a particular direction as shown in FIGS. 6 and 7 and the crystal structure is continuous in the same direction.
A TFT having superior characteristics can be obtained by making the carrier moving direction during an operation coincide with the direction of the continuous crystal growth.